Announce prefix to BGP conditional

[Robert Hass]

Hi
Is it possible to announce additional prefix in BGP (let's say inject
it) conditional ?

My configuration is 3 BGP sessions:

1st BGP session to ISP
2nd and 3rd BGP sessions to customers

I would like to announce default-route (0.0.0.0/0) via BGP only if I
receiving let's say defined 2 prefixes from my ISP.

Is it possible at BIRD ? How I can do this ?

My ISP will NOT announce default route to me, so I cannot simply
re-announce it to my neighbors.

Rob